This document summarizes a method for quantifying plant and city CO2 fluxes from XCO2 imagery using Gaussian plume modeling. It presents a full processing approach including: 1) modeling single/multiple point sources and extended sources with Gaussian plumes, 2) using an optimal estimation method to retrieve plume parameters and flux, 3) incorporating effective wind speed from ancillary data for emission estimates. The method is validated using synthetic XCO2 images generated from model simulations for various sites. Preliminary results show the Gaussian model can accurately characterize realistic plumes and the method can quantify CO2 emissions to within 10-30% for different instrument configurations and noise levels.