A numerical solution is obtained by the finite element method for the Graetz problem of forced convection heat transfer in the channels of plate-type monolith catalysts. For this purpose a generalized model is constructed for the duct geometry, the fully developed laminar velocity profile is calculated. and the heat transfer problem is solved for both uniform wall heat flux or temperature. The results are summarized in interpolated functional forms providing the axial evolution of the Nusselt number. Such formulae represent a prerequisite to the development of design equations for mass-transfer coefficients in plate-type monolith catalysts for the selective catalytic reduction of NOx with ammonia.