Mixing of coaxial jets within a tube in the presence of blockage has been numerically studied. This configuration is encountered during the modelling of flow and heat transfer in CVD (chemical vapour deposition) reactors. For the conditions prevailing in the reactor, the Reynolds numbers are low and flow can be taken to be laminar and incompressible. The unsteady forms of the governing equations have been solved by a finite volume method that can treat complex three-dimensional geometries. The algorithm is a two-step procedure, wherein the first step predicts the velocity field using an assumed pressure field. The second step corrects the fields using a Poisson equation to obtain the pressure corrections. Advection terms have been treated by a hybrid upwind-central difference technique. The computer code developed is fully three-dimensional, though most computations of the present study have been carried out for two-dimensional geometry. Results have been obtained in the form of velocity vector plots, wall shear stress variation on the block and the tube wall, isotherms and temperature profiles. The flow and heat transfer characteristics of jet mixing have been explored in terms of the Reynolds number, the jet velocity ratio, the axial position of the block, and the blockage ratio. The results obtained show that a proper combination of the process parameters can lead to an improved performance of the CVD reactor.