Modeling a complex engineering system, containing a series of energy-based components, is not a trivial task. Marine propulsion system which typically consists of engine, transmission and propeller, ought to be modeled simultaneously in order to predict the real conditions, accurately. This work which is part of a main project entitled “Ship propulsion dynamic and emission”, takes aim at presenting a set of thermodynamic equations applied in GT Power software for modeling a spark ignition engine fueled by liquid natural gas. Moreover, marine engines usually encounter wave loads during a journey, then a transient harmonic condition is implemented on the engine to specify performance and emission compounds. The results show that any variation on engine speed, influences engine power directly. Although harmonic load fluctuation is less than variation of speed, the rate of change of load amplitude is more than rate of change of speed amplitude, meaning that 4% and 8% speed oscillation contributes to 1.4 and 3.2% load oscillation, respectively. In addition, the sensitivity of a natural gas engine to air fuel ratio and control system is also presented.