Reaction: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$. Moles of $CH_4$ = 16g / 16 g/mol = 1 mol. Moles of $O_2$ = 32g / 32 g/mol = 1 mol. Oxygen is the limiting reactant. From stoichiometry, 2 moles of $O_2$ produce 2 moles of $H_2O$. So, 1 mole of $O_2$ produces 1 mole of $H_2O$. Mass of 1 mole $H_2O$ = 18 g.