Methane cells are a type of cell mainly used for producing energy. In FTB beta, they produce 1.5 times the EU of a Hydrogen Cell. In modpacks that use a newer version of GregTech they produce 3 times as much EU as a Hydrogen Cell (45,000 EU).
Methane cells is one of the products that can be obtained by processing food in an Industrial Centrifuge. Industrial centrifuge accepts things like chicken, beef, fish, apples etc.. Rubber Wood gives more methane cells. This is also a decent use for rotten flesh, as it will produce methane, technically being a 'food'.
This video will show how to use a methane cell to power a gas turbine.