The present invention is natural gas steam reforming apparatus for generating an output gas mixture of carbon dioxide and hydrogen. The apparatus is made from two enclosures. A first enclosure contains a source of water, superheated steam, and channels, located within a lower portion of the first enclosure, which contain a water-gas-shift catalyst for converting CO into CO2 and H2. The heat from hot gas flowing through the channels is released into the first enclosure to boil the water to generate the superheated steam. A second enclosure, contained within an upper portion of the first enclosure, includes a steam inlet for receiving the superheated steam from the first enclosure; a combustion chamber; and a reformation chamber. The combustion chamber is used for combusting a portion of the natural gas to generate additional steam, heat, and a hot gas mixture of CO2, CO, and H2. The reformation chamber is used for steam reforming a remaining portion of the natural gas to generate additional hot gas mixture of CO2, CO, and H2. The hot gas mixture is directed through the channels installed in the first enclosure in which the water-gas-shift catalyst converts residual CO into additional CO2 and additional H2, to produce an output gas mixture of carbon dioxide and hydrogen. In the present invention, the first and second enclosures function as a top-to-bottom linear countercurrent heat exchanger.