In a typical biogas plant, methanogens grow anaerobically on...

In a biogas plant, certain bacteria grow anaerobically on cellulosic material present in cow dung to produce a large amount of methane along with carbon dioxide and hydrogen. These bacteria are collectively called methanogens, with Methanobacterium being a common example.

The mixture of gases produced is called biogas, which predominantly consists of methane ($CH_4$), carbon dioxide ($CO_2$), and hydrogen ($H_2$).

Answer: Methane, Carbon dioxide, and Hydrogen