Abstract: In an off-gas recombiner for a nuclear power plant, a preheater, catalyst bed and condenser are arranged in close proximity to each other within an outer shell. Fluid communication between these components is provided by passages of short length and large cross sectional area defined by the outer shell, a semicylindrical vessel liner and a duct or transition piece. In operation, off-gas from a main power plant condenser mixed with dilution steam enters the recombiner vessel, is heated by the preheater, passed through the catalyst bed where free oxygen and hydrogen are recombined into water vapor, and cooled by the condenser where the water vapor is condensed to a liquid. The liquid is returned to the power plant feed water stream while the remaining non-condensable gases are fed to other treatment apparatus. The outer shell is insulated from the heated gases by the stream of inlet off-gas flowing through a passage defined by the outer shell and the vessel liner.