assuming full combustion, CO2 coming out is
* none from the H2
* 1:1 mole ratio from the CH4 

So mass of CO2 =

( mass of CH4 ) x ( molar mass of CO2 ) / ( molar mass of CH4)

Add a slight adjustment for Ethane content of the natgas, if you want.

You also need the energy value of the H2 to figure out the total mass, so you can get the mass of CH4.