With growing environmental pollution and concerns about fossil fuel depletion worldwide, there is an urgent need to find a solution for this problem. Using alternative fuels, such as natural gas, which can burn much cleaner than petrol or gasoline and as an advantage, it’s much cheaper than other conventional fuels and is much more widely available than oil in our planet. The most effective way we can utilize this alternative fuel in the common internal combustion engine, is by means of direct injection technology. Before natural gas can be utilized in common automotive engines, it’s necessary to conduct simulations and thus optimize these engines to maximize output power prior being built. Optimizing engines can only be achieved through simulation. KIVA-3V is a well-accepted engine simulation tool, recognized by industrial users and researchers. KIVA-3V lacks the ability to simulate gaseous fuel injections as it’s only designed to deal with liquid fuels. In this research, researched the governing equations on gas injections and used them to develop a numerical code for KIVA-3V to enable simulation of gaseous injections. We validated our modified version of KIVA-3V with two different sets of experimental data which we previously had. We showed our modified KIVA-3V code can effectively simulate gaseous injections producing very exact results. The gaseous fuel considered in this research is pure methane.