What is the chemical equation for the digestion/decomposition of cellulose?
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 

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