1. A fluid flow simulation method for simulating well system fluid flow in a subterranean region, the method comprising: identifying subsystem models associated with distinct subsystems within a subterranean region, each subsystem model including fluid flow variables and representing well system fluid flow dynamics associated with a sub-region in the subterranean region, the subsystem models being connected by junction models that represent interactions among the subsystem models; for each of the subsystem models, performing, by operation of one or more computers, an elimination of internal variables of the subsystem model to express the internal variables in terms of junction variables of the junction models; solving the junction variables based on the junction models; and solving the internal variables of the subsystem model based on the solved junction variables.