Question

why combustion of methane is a combination reaction?/

Answer 1

It's not. A combination reaction takes two or more smaller molecules and creates one (and only one) larger molecule as a product. A combustion reaction takes a fuel (like methane), reacts it with oxygen, and yields CO2 and H2O as products.
\[N{_2} + 3H{_2} \rightarrow 2NH{_3}\]\[CH{_4} + 2O{_2} \rightarrow CO{_2} +2H{_2}O\]

Answer 2

u are right jfraser but with respect to CO2 u can say it combination reaction