A numerically calculated deposit was used to identify the shape of a fouled tube and an orthogonal curvilinear grid was constructed in and around the deposit. The flow and temperature field was calculated and an implicit boundary condition, accounting for the effects of turbulence, was introduced at the solid-fluid interface for the simultaneous calculation of the temperature held in the fluid and the solid. Validation is presented against experimental and numerical calculations. Finally, a reduction of 8-17% in the heat transfer rate was found to occur for a deposit corresponding to 8 h of operation in a staggered tube bundle heat-exchanger of a lignite utility boiler. Thermal resistance of the deposit was also calculated.