Due to the issues of low flame speed and high CH₄ emissions for a natural gas engine, investigations into the partial oxidation fuel reforming (POFR) method used in natural gas engines to blend H₂ have become increasingly valuable. In this paper, the combustion process, engine performance, and emissions of a natural gas engine with fuel-reforming gases blended together have been numerically studied. The results show that a higher fuel-reforming ratio can effectively improve the engine combustion performance, especially at lean-burn conditions. Combustion with reformed gases can increase the thermal efficiency by almost 2% at the full-load condition, whereas fuel reforming significantly affects the natural gas engine’s power performance. Furthermore, CH₄ and NO_X emissions decrease significantly with increasing fuel-reforming ratio. In conclusion, fuel reforming for a natural gas engine has a promising future in reducing greenhouse gas emissions and improving economic performance.