Methane (CH4) reforming was carried out in an internal combustion engine (an engine reformer). We successfully produced syngas from the partial oxidation of natural gas in the cylinder of a diesel engine that was reconfigured to perform spark ignition. Performing the reaction in an engine cylinder allows some of the exothermicity to be captured as useful work. Intake conditions of 110kPa and up to 480 degrees C allowed low cycle-to-cycle variability (COVnimep <20%) at methane-air equivalence ratios (phi(M)) of 2.0, producing syngas with an H-2-to-CO ratio of 1.4. Spark ignition timing was varied between 45-30 degrees before top-dead-center (BTDC) piston position, showing significant improvement with delayed timing. Hydrogen (H-2) and ethane (C2H6) were added to simulate recycle from a downstream synthesis reactor and realistic natural gas compositions, respectively. Adding these gases yielded a stable combustion up to hydrocarbon-air equivalence ratios (phi(HC)) of 2.8 with COVnimep2.4, but <1mg/L below these equivalence ratios. These results demonstrate that the engine reformer could be a key component of a compact gas-to-liquids synthesis plant by highlighting the operating conditions under which high gas conversion, high H(2-)to-CO ratios close to 2.0, and low soot production are possible.