In order to achieve the requirements on emissions of harmful substances in exhaust gases, when the engine runs with stoichiometric mixture, it is advisable to use a special multifunctional three-way catalyst, specifically designed for complex conversion of exhaust gases produced by a natural gas-powered ICE. A number of catalysts for engine exhaust aftertreatment were identified, which are able to oxidize methane and reduce nitrogen oxides efficiently in a wide temperature range. The key components of three-way catalysts for exhaust gases of natural gas engines are precious metals with Ce, Ce-Zr and other additives to improve efficiency and thermal stability of the contact. Experimental studies have shown that efficient (over 90%) conversion of gas engine exhaust components takes place in the range of excess air ratio from 0.99 to 1.01. Aftertreatment of toxic components of a gas engine with λ>1 requires the use of catalytic converters, which use a modular design concept allowing to build systems that ensure conversion of nitrogen oxides and methane in the presence of excess oxygen. Such systems combine components with oxidation and reduction functions in accordance with the temperature, concentration and toxic components to oxygen ratio. A good solution to reduce NOx emission is an SCR converter using urea. Theoretical studies have shown that the highest efficiency of nitrogen oxides reduction with ammonia is achieved in the temperature range of 400...500 °C and reaches over 97%. This research is performed, a new project with the Ministry of Education and Science of the Russian Federation (the unique identifier for Applied Scientific Research (project) RFMEFI62414X0005).