An integrated fuel reformer-engine system and method provides an on-board fuel reformer for preparing a hydrogen-rich reformate; an engine operating on one or a combination of reformate and liquid fuel, in combination with an exhaust catalyst for treating engine exhaust. The supply of air, reformate, and liquid fuel to the engine and exhaust catalyst is metered so as to provide low hydrocarbon and NOx emissions over a range of operating conditions from cold-start and idle through vehicle road-loads. In one embodiment, the system provides near-zero cold start hydrocarbon and NOx emissions with ultra-lean start using substantially 100% reformate fueling. In another embodiment, accelerated catalyst heating is provided by supplying reformate mixed with engine exhaust to the catalyst. Alternately, an ignition source is employed to ignite the engine exhaust or reformate-exhaust mixture. In another embodiment, the present system provides ultra-low NOx emissions at light and mid loads using extreme dilution and partial reformate fueling.