Increasing air pollution has brought about the search for alternative fuels instead of conventional fuels. It is aimed to make existing internal combustion engines work with alternative fuels with the least structural changes. Natural gas (NG) is one of the most recent alternative fuel studies because it is both cheaper and more environmentally friendly. In this study, it was aimed to minimize the dependence on petroleum-based fuels by enabling an existing compression ignition (CI) engine to operate with spark ignition with NG. For this reason, in heavy-duty diesel engine; It was modeled as 0/1-dimensional with spark plug assembly instead of diesel injector and low-pressure NG fuel injector mounted on the intake manifold. Afterwards, the performance, combustion characteristics, and emission values of the engine, which were converted to NG, were compared with the experimentally validated diesel model. In addition to the comparisons made under similar conditions, the effects of start of combustion (SOC) time and Air/Fuel (A/F) ratio changes in NG use were performed parametrically. In the same conditions, it was observed that the power, fuel consumption, and efficiency of the engine increased in NG fuel use compared to diesel fuel use. However, with the parametric studies in NG use, an improvement of 84.5% was achieved in NOX emission without any performance loss compared to diesel use.