This research paper studied the environmental impact of using methane fuels for supplying internal combustion engines. Methane fuel types and the methods of their use in internal combustion engines were systematized. The knowledge regarding the environmental impact of using methane fuels for supplying internal combustion engines was analyzed. The authors studied the properties of various internal combustion engines used for different applications (specialized engines of power generators—Liebherr G9512 and MAN E3262 LE212, powered by biogas, engine for road and off-road vehicles—Cummins 6C8.3, in self-ignition, original version powered by diesel fuel, and its modified version—a spark-ignition engine powered by methane fuel) under various operating conditions in approval tests. The sensitivity of the engine properties, especially pollutant emissions, to its operating states were studied. In the case of a Cummins 6C8.3 modified engine, a significant reduction in the pollutant emission owing to the use of methane fuel, relative to the original self-ignition engine, was found. The emission of carbon oxide decreased by approximately 30%, hydrocarbons by approximately 70% and nitrogen oxide by approximately 50%, as well as a particulate matter emission was also eliminated. Specific brake emission of carbon oxide is the most sensitive to the operating states of the engine: 0.324 for a self-ignition engine and 0.264 for a spark-ignition engine, with the least sensitive being specific brake emission of nitrogen oxide: 0.121 for a self-ignition engine and 0.097 for a spark-ignition engine. The specific brake emission of carbon monoxide and hydrocarbons for stationary engines was higher in comparison with both versions of Cummins 6C8.3 engine. However, the emission of nitrogen oxide for stationary engines was lower than for Cummins engines.