NOVELTY - Operating a methane-fuelled engine and treating exhaust gas coming from the engine comprises fuelling the engine with a lean fuel mixture when operating the engine at first set(s) of points on an engine map; fuelling the engine with a rich fuel mixture when operating the engine at one of a predetermined second set of points on the engine map; and flowing an exhaust gas from the engine through a methane oxidation catalyst.    USE - The invention is useful for operating methane-fuelled engine and treating exhaust gas coming from engine, i.e. prime mover engine for a vehicle or machine (claimed).    ADVANTAGE - The invention provides improved performance and lowers cost. It provides lower emission benefits with lean bum spark ignition (LBSI) methane-fuelled engines, while reducing emissions of unburned methane by managing the performance and de sulfation of a methane oxidation catalyst.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for methane-fuelled engine (100) comprising an intake air manifold defining a passage through which air can flow into a combustion chamber of the engine; a fuel metering valve operable to regulate the mass flow rate of a fuel comprising methane introduced into the combustion chamber through a fuel supply pipe; a throttle disposed inside the intake air manifold for regulating the mass flow rate of air that is introduced into the combustion chamber; an exhaust manifold defining a passage that is in communication with the combustion chamber for receiving combustion products from the combustion chamber and directing the combustion products to an exhaust pipe; a methane oxidation catalyst disposed in the exhaust pipe; sensor(s) associated with the methane fuelled engine for calculating or measuring lambda in the exhaust manifold or in the intake air manifold; and an electronic controller programmed to operate the engine in one of a lean bum mode and a fuel-rich mode at respective predetermined points on an engine map.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of apparatus used in the method.    Methane-fuelled engine (100)    Fuel injection system (110)    Port injection (112)    Fuel supply pipe (114)    Intake port (116)    Intake air manifold (118)    Throttle (119)    Engine combustion chamber (120)    Piston (124)    Cylinder head (126)    Plug (128)    Catalytic converter (140)