We prove that the time-dependent solutions of a large class of Smoluchowski
coagulation equations for multicomponent systems concentrate along a particular
direction of the space of cluster compositions for long times. The direction of
concentration is determined by the initial distribution of clusters. These results allow
to prove the uniqueness and global stability of the self-similar profile with finite mass
in the case of coagulation kernels which are not identically constant, but are constant
along any direction of the space of cluster compositions.