What is the chemical equation for the digestion/decomposition of cellulose? I'm trying to find the chemical equation/pathway from cellulose $\ce{(C6H10O5)_n}$ to methane $\ce{CH4}$. What are the other reactants or products in the reaction?

$$\ce{C6H10O5 + ? -&gt; CH4 + ?}$$ 

Everything I've found talks about bacteria and enzymes (cellulase, mostly) in terms of the process of enteric fermentation, but I've yet to find a single reaction or series of reactions for this conversion. At least one product would have to have oxygen in it, but that really hasn't narrowed down my search.

I'm mainly interested in how methane is created from the diets of herbivores, so it's very possible that I've been researching the wrong reactants from the beginning.
Biologically, methane is produced by methanogens, which are Archeabacteria. They do not utilize carbohydrates such as cellulose or glucose directly. Instead, they typically consume acetate or a mixture of hydrogen gas and carbon dioxide. These stoichiometry of these two processes is

$\ce{4H2 + CO2 -&gt; CH4 + 2H2O}$ or

$\ce{CH3C(O)OH -&gt; CH4 + CO2}$.

To get overall conversion of cellulose to methane, the cellulose is first fermented by the enteric bacteria you read about, which can produce acetate, hydrogen and carbon dioxide by a variety of pathways. These products are then utilized by the methanogens. One example of an enteric fermentation would be

$\ce{C6H10O5 + 3H2O -&gt; 2CH3C(O)OH + 4H2 + 2CO2}$.

Combining this with the methanogenesis reactions, we can write the net equation of 

$\ce{C6H10O5 + H2O -&gt; 3CH4 + 3CO2}$.