Energy and exergy analyses are carried out for hydrogen production via Gas Turbine Modular Helium Reactor/Organic Rankine Cycle (GTMHR/ORC) system coupled with a proton exchange membrane electrolyzer. A comprehensive parametric study is performed and the effects of some significant variables as compressor pressure ratio, turbine inlet temperature, evaporators' temperature, pinch point temperature difference in the evaporators and degree of superheat at the ORC turbines inlet on the exergy efficiency, rate of produced hydrogen and sustainability index of the integrated system are investigated. Using direct search method by the Engineering Equation Solver (EES) software, the combined system is optimized to achieve the maximum exergy efficiency. Results show that the exergy efficiency difference between the combined system and GTMHR cycle increases with an increase in pressure ratio. Also, it is observed that the rate of produced hydrogen increases with increasing turbine inlet temperature and takes the maximum value with change in evaporators' temperature. Under the optimized condition, exergy efficiency, rate of the produced hydrogen and sustainability index of the proposed combined system are Calculated to be 49.21%, 56.2 kg/h and 1.972, respectively. (C) 2016 Elsevier Ltd. All rights reserved.