Soft-sphere discrete element method and Navier-Stokes equations were coupled with the equations of energy to simulate the agglomeration process at high temperature in an air-polyethylene fluidized bed. The inter-particle cohesive force was calculated based on solid bridging by the viscous flow. The agglomerates were tracked as a real object with irregular shapes in which their translational and rotational motions were calculated according to the multi-sphere method. Fluidization behavior of an agglomerating bed was successfully simulated in terms of increasing the size of agglomerates as well as decreasing the bed height. The influence of gas velocity on the size of agglomerates was investigated and it was shown that the formation of massive agglomerates is postponed as the gas velocity is increased. Microscopic mechanistic studies in terms of contact time, cohesive force and repulsive force, which are the key parameters in the formation of agglomerates, were also carried out. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.