The Combustion of natural gases differently composed from almost pure methane until low-calorific natural gases (LCNG) with high molar fraction of inert gas (nitrogen) in gas engines is the main subject of this work. The paper presents the main thermodynamic properties of laminar flames calculated for natural gases (NG) differing in molar composition, such as flame temperature, velocity of flame propagation and flame thickness. Also presented is a visualization of the propagation of two types of flames in the constant volume chamber: a classical laminar flame propagated in uniform premixed mixture and a turbulent flame created by ignition through a jet of post-flame gases from ignition in prechamber. The paper presents the results of research of combustion of high molar fraction of methane NG and of strongly nitrified NG (only 38.2% of methane) in gas engines equipped with ignition prechamber. A strong correlation between emission of pollutants and equivalence ratio was found.