Polygeneration is the coupling of energy conversion and conversion towards useful chemicals, providing a route towards more flexible and efficient energy systems. In this work, we explore a particular concept of polygeneration using an internal combustion engine as a reactor for partial oxidation to generate synthesis gas in variable combinations with mechanical work and heat. Experiments were performed in a single-cylinder engine operated in homogeneous-charge compression-ignition (HCCI) mode on a mixture of methane and air with dimethyl ether (DME) as a reactivity-enhancing additive. For intake temperature from 100 to 190 degrees C, the range of stable, non-sooting operation with acceptable pressure-rise rates was determined in terms of equivalence ratio and DME mole fraction in the fuel. At 150 degrees C intake temperature, 8.7-9.5% DME were needed to stabilize operation at equivalence ratios between about 1.3 and 2.7. Experimental results from fuel-rich conditions with equivalence ratios ranging from 1.65 to 2.34 were compared to simulations with a homogeneous, single-zone engine model. The concept of exergy was used to investigate the thermodynamic performance of the polygeneration engine. The effect of the equivalence ratio on work and heat output, thermal and exergetic efficiency, and selectivity towards useful product species was investigated. In the experiments a work output of up to 160 J (phi=1.65) per cycle (IMEP=4.82 bar) and exergetic efficiencies of up to 81.5% (phi=2.34) were achieved. The simultaneous generation of synthesis gas had a selectivity of up to 72% for hydrogen and 79% for carbon monoxide (both at phi=2.34).