Due to ecological and economic advantages, natural gas is used as an alternative fuel in the transportation sector in the form of compressed natural gas (CNG) and liquefied natural gas (LNG). Development of infrastructure is necessary to popularize vehicles that use alternative fuels. Selected positive factors from EU countries supporting the development of the CNG market were discussed. The process of natural gas vehicle (NGV) fast filling is related to thermodynamic phenomena occurring in a tank. In this study, the first law of thermodynamics and continuity equations were applied to develop a theoretical model to investigate the effects of natural gas composition on the filling process and the final in-cylinder conditions of NGV on-board composite cylinder (type IV). Peng–Robinson equation of state (P-R EOS) was applied, and a lightweight composite tank (type IV) was considered as an adiabatic system. The authors have devised a model to determine the influence of natural gas composition on the selected thermodynamic parameters during fast filling: Joule–Thomson (J-T) coefficient, in-cylinder gas temperature, mass flow rate profiles, in-cylinder mass increase, natural gas density change, ambient temperature on the final natural gas temperature, influence of an ambient temperature on the amount of refueled natural gas mass. Results emphasize the importance of natural gas composition as an important parameter for the filling process of the NGV on-board composite tank (type IV).