A system includes a main engine operating on gaseous fuel from a tank. An auxiliary engine operates using gaseous fuel vented from the tank. A first exhaust passage receives a first stream of exhaust gas from the main engine. A second exhaust passage receives exhaust gas from the auxiliary engine, which passes through an ammonia-producing catalyst. A third exhaust passage is fluidly connected to the first and second exhaust passages and routes exhaust gas through a NOx reducing catalyst. The system operates such that a mass flow of ammonia generated by the ammonia-producing catalyst substantially matches a total mass flow of NOx gas in the third exhaust passage that is treated within the ammonia-producing catalyst.