One of important problems in converting diesel engines to natural gas is an efficiency drop, caused, in particular, by lower compression ratio. Studies conducted at NAMI have shown possibility of substantial (10...15%) improvement of fuel economy at partial load conditions when running with stoichiometric mixtures, using the Miller cycle and maintaining the same geometric compression ratio as in the diesel prototype engine. Further increase of the gas engine's efficiency is possible through leaning of the combustion mixture at partial loads, however, it creates a problem of increased NOx emissions, because three-way catalyst is not efficient for NOx at lean-burn operation. Thus, to develop new gas engines with high energy efficiency and meeting future emission standards, it is necessary to develop complex exhaust gas aftertreatment systems to treat the toxic components efficiently when the engine runs on stoichiometric and lean mixtures. This paper discusses issues related to creation of such complex systems.