Abstract: A heat exchanger for simultaneously vaporizing a hydrocarbon fluid and water to produce a hydrocarbon fuel gas and steam includes a body member with a smooth surface axial bore having an enlarged end. The non-enlarged portion of the bore receives an elongated thick-walled sleeve having two separate coextensive, interjacent, helical passages formed in its outer surface and positioned in liquid tight engagement with the inner surface of the bore. The first passage communicates with a water inlet and steam outlet at opposite ends of the body member. The second passage is of larger in cross section than the first and communicates at one end with a hydrocarbon fluid inlet on the body member adjacent the water inlet and at its other end with the enlarged bore portion, the latter having a hydrocarbon fuel gas outlet.