Heat transfer in magnetohydrodynamic flow of a liquid metal in rectangular ducts with thin conducting walls in the presence of a large transverse magnetic field is examined. A significant fraction of the fluid flow, in the form of high velocity jets, is confined within the boundary layers (side layers) adjacent to the side walls which are parallel to the magnetic fields. The existence of the high velocity jets may drastically enhance the cooling capacity. Two schemes - integral and explicit - for the treatment of the flow in the side layers are discussed. Heat transfer calculations based on these schemes are compared. Work supported by the U.S. Department of Energy/Office of Fusion Energy.