How to find the volume of the products in methane combustion?
One mole $\ce{CH4}$ reacts with 2 moles $\ce{O2}$, producing 1 mole $\ce{CO2}$ and 2 moles $\ce{H2O}$. 
So 1 liter $\ce{CH4}$ produces 1 liter $\ce{CO2}$ and 2 liter $\ce{H2O}$, if $\ce{H2O}$ is taken in the gaseous phase.