Once-through steam generators (OTSGs) produce steam by recycling produced water in the steam-assisted-gravity-drainage (SAGD) process for extracting Alberta's oil sands via in situ recovery of bitumen. The industry's specifications for water quality, steam quality, and water recycle ratio, as well as high temperature and pressure operations, lead to the fouling of OTSG tubes, which has important economic, safety, and environmental consequences. The roles and mechanisms of inorganic and organic impurities on tube fouling also complicate the study of heat transfer in OTSGs. Heat transfer in OTSGs involves radiation, convection, conduction, forced convection, and boiling. In this investigation on OTSG fouling, a steady-state mathematical model was built in Microsoft Excel, incorporating industry design data, to demonstrate the impact of tube fouling on heat transfer and tube-wall temperature. The model predictions indicate that tube fouling introduces a significant thermal resistance that not only decreases the rate of heat transfer but also causes an increase in the tube-wall temperature such that it could exceed the safe operating limits of the tube material, leading to potential tube failures.Predictions are provided for the average tube-wall temperature exceeding the recommended operating limit of 700 K over a range of foulant thermal conductivity (ie, 0.05-2 W m(-1)K(-1)) and foulant thickness (ie, 0.4-11 mm corresponding to 0.1%-30% of the tube radius).