A process model has been developed to evaluate the potential performance of a large-scale high-temperature co-electrolysis plant for the production of syngas from steam and carbon dioxide. The co-electrolysis process allows for direct electrochemical reduction of the steam/carbon dioxide gas mixture, yielding hydrogen and carbon monoxide, or syngas. The process model has been developed using the UniSim system-analysis code. Using this code, a detailed process flow sheet has been defined that includes all the components that would be present in an actual plant such as pumps, compressors, heat exchangers, turbines, and the electrolyzer. Since the electrolyzer is not a standard UniSim component, a custom integral co-electrolysis model was developed for incorporation into the overall UniSim process flow sheet. The integral co-electrolysis model assumes local chemical equilibrium among the four process-gas species via the water-gas shift reaction. The electrolyzer model allows for the determination of co-electrolysis outlet temperature, composition (anode and cathode sides); mean Nernst potential, operating voltage and electrolyzer power based on specified inlet gas flow rates, heat loss or gain, current density, and cell area-specific resistance. The integral electrolyzer model was validated by comparison with results obtained from a fully three-dimensional computational fluid dynamics model developed using FLUENT, and by comparison with experimental data. This paper provides representative results obtained from the UniSim flow sheet model for a 300 MW, co-electrolysis plant, coupled to a high-temperature gas-cooled nuclear reactor. The co-electrolysis process, coupled to a nuclear reactor, provides a means of recycling carbon dioxide back into a useful liquid fuel. If the carbon dioxide source is based on biomass, the overall process, from production through utilization, would be climate-neutral. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.