A substantial increase of the concentration of non-condensable gases in the mixture with steam can occur in a non-vented pipeline due to the condensation. This phenomenon is investigated with the thermal-hydraulic and physicochemical code HELIO. The hydrogen and oxygen accumulation is simulated and analyzed for a real non-vented steam pipeline of the nuclear power plant. The results show the propagation of non-condensables concentration front, the temperature and velocity field of the steam-non-condensables mixture, and the velocity and thickness of the condensate that drains on the pipeline's inner walls. The gas mixture temperature is verified with measurements from a full size test facility. The presented modelling approach and numerical results are unique regarding the simultaneous solution of the heat and mass transfer in the system consisting of the steam-non-condensable gases mixture and the thin liquid film on the pipe's wall. (c) 2006 Elsevier Ltd. All rights reserved.