The invention relates to reactor systems and processes for producing synthesis gas. In one embodiment, a reactor comprises a first reaction zone comprising a combustion catalyst, wherein the first reaction zone is operated at conditions sufficient to produce a combustion zone product comprising heat generated by the combustion of a fuel. The reactor further comprises a second reaction zone comprising a partial oxidation catalyst, wherein the second reaction zone is adapted to receive the combustion zone product and a reaction feed comprising a hydrocarbon gas. The second reaction zone is operated at conditions sufficient for partially oxidizing the hydrocarbon gas to a product stream comprising synthesis gas. The heat of combustion is transferred directly to the reaction feed by mixing it in a feed mix region located in the reactor between the first and second reaction zones, in such a manner that the reaction feed does not need preheating prior to entering the partial oxidation zone.