A Monte-Carlo code has been developed that can be used to optimally design vapor transport systems for isotope-separator-on-line-based radioactive ion beam facilities in lieu of costly iterative trial and error design methods. The code provides a powerful means for delineating diffusion-release and effusive-flow (molecular-flow) processes, in combination, the delay times of which are principal intensity limiters of short-lived radioactive species at such facilities. The code provides time dependent particle evacuation, average distance traveled per particle, and particle/wall interaction information during particle transit through a given vapor-transport system under molecular-how conditions, independent of the chemistry between particles of interest and the materials of which the transport system are constructed; In addition, the code provides powerful graphical insight via particle trajectories that serve as strong assets in arriving at a final design by identifying regions within the transport system where hold-up times are problematical. In this article, we compare simulation and experimental measurement results for transport of noble gases through selected vapor-transport systems using both cosine and isotropic particle re-emission distributions about the normal to the surface following adsorption (isotropic re-emission distributions are-found to be in close agreement with experimental measurements) and describe a concept vapor-transport system that reduces transport times over those of conventional systems by >two orders of magnitude. (c) 2005 American Vacuum Society.