Given $k\ge 2$, for which polynomials $P\in \mathbb{Z}[X]$ does there exist a function $h:\mathbb{N}\rightarrow\mathbb{N}$ with $h^{(k)}(n)=P(n)$?
To determine for which polynomials \( P \in \mathbb{Z}[X] \) there exists a function \( h: \mathbb{N} \rightarrow \mathbb{N} \) such that \( h^{(k)}(n) = P(n) \) for \( k \ge 2 \), we need to analyze the behavior of \( P \) and the implications for \( h \).

1. **Case \( P(x) = x + c \):**
   - Suppose \( P(x) = x + c \). Then, we need \( h^{(k)}(x) = P(x) = x + c \).
   - We can write \( h^{(k+1)}(x) = h(h^{(k)}(x)) = h(x + c) \).
   - Since \( h^{(k)}(x) = x + c \), we have \( h(x + c) = h(x) + c \).

2. **Injectivity and Surjectivity:**
   - Clearly, \( h \) must be injective because if \( h(a) = h(b) \), then \( h^{(k)}(a) = h^{(k)}(b) \), which contradicts \( h^{(k)}(a) = a + c \) and \( h^{(k)}(b) = b + c \) unless \( a = b \).
   - We claim that \( k \mid c \) is both necessary and sufficient.
   - **Sufficiency:** If \( k \mid c \), then \( c = km \) for some integer \( m \). Define \( h(x) = x + \frac{c}{k} = x + m \). Then:
     \[
     h^{(k)}(x) = h(h^{(k-1)}(x)) = h(x + (k-1)m) = x + km = x + c = P(x).
     \]
   - **Necessity:** Assume \( h(x) \) is surjective modulo \( c \). This implies that \( \{h(1), h(2), \ldots, h(c)\} \) is a complete residue system modulo \( c \). Define \( d(x) = h(x) - x \). Then \( d(x) = d(x + c) \).

3. **Summation Argument:**
   - Consider the sum \( \sum_{j=1}^c h^k(j) \):
     \[
     \sum_{j=1}^c h^k(j) = \sum_{j=1}^c (j + km) = \sum_{j=1}^c j + k \sum_{j=1}^c d(j).
     \]
   - Since \( h^{(k+1)}(j) = h(j) + c \), we have:
     \[
     \sum_{j=1}^c h^{(k+1)}(j) = \sum_{j=1}^c (h(j) + c) = \sum_{j=1}^c h(j) + c^2.
     \]
   - Equating the two expressions, we get:
     \[
     \sum_{j=1}^c d(j) = \frac{c^2}{k}.
     \]
   - Since \( c \mid \sum_{j=1}^c d(j) \), it follows that \( c \mid \frac{c^2}{k} \), implying \( k \mid c \).

4. **General Polynomial \( P(x) \):**
   - If \( P(x) \) is not of the form \( x + c \) and is positive for \( x \in \mathbb{N} \), consider the behavior of \( P \).
   - For all \( x \), either there exists \( m \) such that \( P^m(x) = x \), or \( x, P(x), P(P(x)), \ldots \) is injective.
   - Since \( \mathbb{N} \setminus \text{Im}(P) \) is infinite, there are infinitely many \( x \) such that \( x \) is the root of a chain \( x \rightarrow P(x) \rightarrow \cdots \).

5. **Handling Cycles and Chains:**
   - If \( P^m(x) = x \) has no solutions where \( P(x) \neq x \), we can define \( h(x) = x \) for all \( P(x) = x \) and interweave other chains.
   - For chains \( x_1 \rightarrow P(x_1) \rightarrow \cdots \rightarrow x_k \rightarrow P(x_k) \rightarrow \cdots \), if \( P(x_i) = x_{i+k} \), then \( h(x_i) = x_{i+1} \) works.
   - If \( P^m(x) = x \) and \( \gcd(m, k) = 1 \), there exists \( ma \equiv k \pmod{1} \). Set \( h(x_i) = x_{i+k} \).

6. **Complexity for Non-Coprime \( m \) and \( k \):**
   - If \( \gcd(m, k) \neq 1 \), the situation becomes more complex and may not have a straightforward solution.

The final answer is \( \boxed{ P(x) = x + c } \) where \( k \mid c \).