An exhaust system for use with a spark- ignited gaseous fuel internal combustion engine comprising a turbocharger having a turbine in exhaust receiving communication with the engine and a compressor in charge air providing communication with the engine, the system comprising a main exhaust gas line in exhaust gas receiving communication with the turbine; a three-way catalyst positioned within the main exhaust line, the three-way catalyst configured to oxidize carbon monoxide and methane, and reduce nitrogen oxides present in the exhaust gas flowing through the main exhaust gas line; an exhaust gas recirculation line in exhaust gas receiving communication with the main exhaust line and exhaust gas providing communication with air flow received by the compressor, the exhaust gas recirculation line being positioned downstream of the three-way catalyst.