synthetic natural environments

isn't that kind of a contradiction in terms?

Depends on your definition of synthetic:

http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=synthetic

An environment that was cobbed together from various natural elements that didn't grow together naturally, such as a ponderosa pine forest with palm tree lined lagoons next to it might be considered a synthetic natural environment. Probably not what they're getting at here. This is probably more something like a computer generated environment that had computer simulated elements based on natural things like trees and hills and streams rather than an environment that was nothing but floors and walls.