This paper provides a comparison of simulation results with experimentally measured performance data in a lean-burn engine operating with natural gas over a wide range of air-fuel ratios. The Ricardo WAVE 3.3 engine simulation code was used to predict intake and exhaust manifold pressures and flow rates, as well as cylinder pressure and engine performance parameters. Experimental data were obtained from a Ricardo Hydra single-cylinder research engine operating in a speed range of 1000 - 3000 rpm. Calculations were conducted with both a simplified mass burn-rate model, as well as a more sophisticated engine combustion model. Comparison between the two different combustion models and experimental data showed that both models were able to predict the engine performance with satisfactory accuracy under stoichiometric conditions. With increasing air-fuel ratio up to the lean limit, however, the more complete combustion model resulted in closer agreement with the experimental results.