In this work, a fully three-dimensional (3D) model of combined computational fluid dynamics and discrete element method (CFD-DEM) is for the first time developed to study the gas-solids flow and heat transfer in fluidized beds with an immersed tube. A critical bed thickness is first determined at which the bed can be regarded as fully 3D. Then the validity of the model using the critical bed thickness is tested both qualitatively and quantitatively. It is shown that the model can successfully reproduce the typical relationship between pressure drop and gas velocity, and flow and heat transfer characteristics such as the four distinct stages of bubble transit through the tube and the peak of heat transfer coefficient between tube and the bed for certain gas velocity (which are however not well-predicted by previous 2D CFD-DEM and 2D CFD-3D DEM models). Finally the results are analyzed to improve the fundamental understanding of the system. It is demonstrated that both the gas and solids phases have 3D flow characteristics including the unique feature of 3D orientations of gas velocity vector field around the bubble. It is predicted that the maximum heat transfer coefficient is a result of the competition between surface particle conduction and surface-fluid convection. The obtained results should be useful to the development of the fundamental understanding of the flow and heat transfer characteristics in a fluidized bed with immersed tubes. (C) 2016 Elsevier Ltd. All rights reserved.