This study proposes an efficient and accurate methodology that combines detailed chemical kinetics with a validated multi-zone thermodynamic model to calculate the emissions of a spark-ignition, natural gas fueled engine. The relative air-fuel ratio has been changed from 0.8 to 1.53, and the nominal brake mean effective pressure has been varied from 0.2 MPa to 1.29 MPa. The multi-zone model in-cylinder pressure and temperature traces, as well as other experimental engine quantities, have been considered as input data for the chemical submodel. The objective has been to reproduce and interpret the measured engine-out data in order to obtain insight into the in-chamber combustion and pollutant formation processes. The concentrations of nitric oxide, carbon monoxide, hydrogen, carbon dioxide and hydrocarbon, as well as the oxygen concentration after ignition, have been compared with experimental data and some of them have been compared with the results of conventional models. The model results, based on detailed chemistry simulation, have been found to be in good agreement with experimental data for all the species at engine exhaust, and a higher prediction capability than that obtained through simplified reaction and chemical equilibrium methods has been shown. The influences of the uncertainties in RAFRs and unburned mass fractions on the calculated results are discussed. The unburned gas fraction, derived from the calculated and measured oxygen concentration at the engine exhaust, has been shown to be a way of correcting the hydrocarbon emissions.