Find all polynomials $P(x)$ with integer coefficients, such that for all positive integers $m, n$, $$m+n \mid P^{(m)}(n)-P^{(n)}(m).$$

[i]Proposed by Navid Safaei, Iran[/i]
To solve the problem, we need to find all polynomials \( P(x) \) with integer coefficients such that for all positive integers \( m \) and \( n \), the following condition holds:
\[ m+n \mid P^{(m)}(n) - P^{(n)}(m). \]

1. **Constant Polynomial Case:**
   - If \( P(x) \equiv c \) where \( c \in \mathbb{Z} \), then \( P^{(m)}(n) = c \) and \( P^{(n)}(m) = c \).
   - Therefore, \( P^{(m)}(n) - P^{(n)}(m) = c - c = 0 \).
   - Since \( m+n \mid 0 \) is always true, constant polynomials \( P(x) \equiv c \) satisfy the condition.

2. **Non-Constant Polynomial Case:**
   - Assume \( \deg P > 0 \). We use the fact that \( x - y \mid P(x) - P(y) \) for any polynomial \( P(x) \).
   - The given condition can be rewritten as:
     \[ m+n \mid P^{(m)}(n) - P^{(n)}(m). \]
   - For \( m > n \), we can write:
     \[ m \mid P^{(m-n)}(n) - P^{(n)}(n). \]
   - Define \( T(m, n) \) as the smallest positive integer such that there exist positive integers \( a \) and \( b \) with \( m \mid P^{(a)}(n) - P^{(b)}(n) \) and \( |a - b| = T(m, n) \).

3. **Minimality Argument:**
   - If \( m \mid P^{(a)}(n) - P^{(b)}(n) \) and \( |a - b| = c \), then \( T(m, n) \mid c \).
   - Write \( c = kT(m, n) + b \) with \( 0 \leq b < T(m, n) \). By the minimality of \( T(m, n) \), we must have \( b = 0 \).

4. **Claim: \( T(m, n) \mid m \):**
   - From \( Q(m, n): m \mid P^{(m-n)}(n) - P^{(n)}(n) \) and \( Q(2m, n): m \mid 2m \mid P^{(2m-n)}(n) - P^{(n)}(n) \),
     \[ m \mid P^{(2m-n)}(n) - P^{(m-n)}(n). \]
   - Hence, \( T(m, n) \mid (2m - n) - (m - n) = m \).

5. **Prime Case:**
   - For every prime \( q \) and \( n < q \), we have \( T(q, n) = 1 \) or \( q \).

6. **Large \( n \) Case:**
   - For large enough \( n \), at least one of the following equalities must hold:
     \[ P^{(n)}(-n) = P^{P(n)}(-P(n)), \]
     \[ P^{(n)}(-n) = P^{-P(n)}(P(n)), \]
     \[ P^{(2n)}(-n) = n. \]

7. **Contradiction for Non-Constant Polynomials:**
   - If \( P \) has a positive leading coefficient, then \( \lim_{n \to \infty} P(n) = +\infty \).
   - If \( P \) has a negative leading coefficient, then \( \lim_{n \to \infty} P(n) = -\infty \).
   - Using the properties of polynomials and the minimality argument, we can show that the conditions lead to contradictions for non-constant polynomials.

8. **Conclusion:**
   - The only polynomials that satisfy the given condition are constant polynomials \( P(x) \equiv c \) where \( c \in \mathbb{Z} \).

The final answer is \( \boxed{ P(x) \equiv c } \) where \( c \in \mathbb{Z} \).