Sponsorship: Department of Energy, Los Alamos National Laboratory. This document describes the methodology used to predict single-event upset rates for Xilinx Virtex FPGAs based on the CREME96 orbit modeling tool. Using this methodology, SEU upset rates are obtained for several Xilinx Virtex FPGAs including Virtex, Virtex-II, and Virtex-4. Further, SEU upset rates are obtained for a variety of orbits for each of these FPGAs.