The so-called condensed mode operation is more fashionable at present in the gas-phase fluidized-bed ethylene polymerization process because of a substantial increase in the space-time yield of polymer production. However, the instability of the fluidized bed in condensed mode operation has never been analyzed theoretically before regardless of its critical importance of commerce. The mechanism of "wet state instability" was first proposed in this paper for the instability analysis due to agglomeration of fluidizing particles. The absorption of heavy components on the polymer particles was assumed to make particles stickier by formation of a liquid bridge. On the basis of a force balance of the agglomerate, a novel model has been developed to predict the maximal mole fraction of the condensed agent (isopentane) permissively existing in the fluidizing gas stream and then to distinguish the unstable operating zone of the fluidized bed. It is found that the theoretical calculation agrees with the industrial operation data very well. Moreover, a criterion was recommended to identify the transition of polymer powders from group B, through A, to group C, in which the bed layer would be collapsed because of the excessive cohesive forces acting among the particles.