The current document is directed to aggregation of computers into distributed computing systems and, in particular, to methods and systems that simulate computer-system aggregations to test computer-system-aggregation management subsystems. In a described implementation, a host-gateway appliance is modified to simulate host-server-computer responses to management commands and requests. In this implementation, a replicated and modified management-server database is used both by a management server under test as well as by the modified host-gateway appliance.