What accuracy are you looking for?

You will not be too far wrong by adding up the mass flow of intake air and fuel supply and simply using the properties of air for the specific heat. Air is 78% nitrogen and exhaust gas is not too different from that ... The water component only makes a meaningful interference if you are condensing it, and if you are using the exhaust to boil something else (water?) you're not going to get the exhaust gas temperature below the boiling point inside the boiler.

Thank you BrianPetersen for your response,

Then without that unknown composition of exhaust gas, would you please to give me the example approximation formula to determine specific heat of exhaust gas?

I've seen the figure 1.063 kJ/kg.K used pretty frequently, although I've never looked into what assumptions are behind it. That's about 6% higher than regular air (1.006 kJ/kg.K).

oh duh, that's plain air at an elevated temp vs. at room temp.

Gas engine, so spark ignition?.
Therefor the exhaust gases will be only Nitrogen, Carbon Dioxide and Water (ok plus a little Argon).
Go the the fuel for the ratio of Hydrogen to Carbon, then do an element balance for those amounts. Light fuels such as propane and natural gas will have more water and less CO2, heavy fuels the other way around
Tables are available for specific heats on all of those elements and compounds, use the partial pressure relationships to get the specific heat of the mixture.

To get mass flow measure the mass of air going in and mass of fuel also going in, it is going to be the same coming out...

Hydrae