The invention relates to a chemical reactor and reformer tubes for reforming a first feed stream comprising a hydrocarbon gas and steam. The chemical reactor comprises a shell with a heat source and one or more reformer tubes. The reformer tube is arranged to house catalyst material and is arranged to being heated by the heat source. The reformer tube comprises a first inlet for feeding said first feed stream into a first reforming reaction zone of the reformer tube, and a feed conduct arranged to allow a second feed stream into a second reforming reaction zone of the reformer tube. The second reforming reaction zone is positioned downstream of the first reforming reaction zone. The feed conduct is configured so that the second feed stream is only in contact with catalyst material in the second reforming reaction zone. The invention also relates to a process of producing CO rich synthesis gas at low S/C conditions.