CNG, or compressed natural gas, is a clean alternative fuel for transport vehicles. CNG offers several advantages, including as lower expenses, less emissions, and simpler vehicle maintenance. Natural gas vehicles, or NGVs, are vehicles equipped with high-pressure tanks that hold compressed natural gas (CNG). This work presents a mathematical simulation model to investigate the fast-filling procedure for buffer storage at CNG refueling stations. To represent the process, the conservation equations of mass, momentum, and energy are discretized and solved in an iterative manner. Heat transfer between the vehicle tank and the environment prevents the temperature rise so more gas mass could be filled to the tank. The heavier gases in the gas mixture cause the mass to be higher. This can be a disadvantage for fuel quality, but it seems to be useful for the filling process.