A method of modifying reformate gas composition downstream of the reformer so that it remains in a carbon-free region, i.e. above the critical oxygen/carbon ratio even at the desired minimum operating temperature. This is accomplished by using staged partial oxidation in a partial oxidation chamber downstream of the reformer. A small amount of air (or oxygen) is added. The net result is an increase in the oxygen/carbon ratio and, thus, a lowering of the required temperature for soot-free operation. Immediately downstream of this partial oxidation chamber, heat can be removed to cool the gas prior to a second stage of partial oxidation. A second stage partial oxidation chamber produces additional water and further increases the oxygen/carbon ratio and further lowers the required gas temperature for soot-free operation. Further stages of partial oxidation followed by cooling of the gas can be repeated until the oxygen/carbon ratio is sufficiently high to allow soot-free operation at the lowest required operating temperature.