Additional info: There are around 28 species of unburned hydrocarbons under this group all with different molecular weight. Dealing separately is time taking and if there is any approximation or correlation, it is much appreciated.
It depends on how accurate the result has to be. The standard procedure is to assume that the unburned hydrocarbon in the exhaust is identical to the fuel hydrocarbon. In your case that would correspond to using the average molecular weight of the hydrocarbon content in the CNG.