||Many applications like fuel reformers, cracking furnaces, fuel cells have the shell and tube configurations, typically consisting of a large number of thin long channels having chemical reactions inside the channels. The channels exchange heat with the fluid in the outer domain. In order to have accurate numerical predictions, it is required to have detailed chemistry calculations inside the channels. However, the number of channels in these types of applications is generally very high. Therefore, the resolution of detailed chemistry inside the thin pipes with three dimensional CFD computations is computationally expensive. In the current work, a methodology is developed for coupling the one dimensional (1D) solution inside the non-permeable channels with the 3D outer flow in shell and tube type of configurations. In the proposed lower dimensional model, called the channel model, the 1D channels have detailed reactions while the outer 3D flow can be reactive or non-reactive. The channel is discretized into 1D grid points and a parabolic solver is used to solve the species transport and energy equations inside the channel. The channel is assumed to have a plug flow but the flow and the mixture properties inside the channel can have the axial variations. Since, the channel walls are non-permeable; the two zones are coupled only through the heat transfer. ANSYS R15 was used to perform the simulations.