1. A method for modeling critical-dimension (CD) scanning-electron-microscopy (CD-SEM) extraction during photolithography process model calibration, the method comprising: receiving a measured CD value which was obtained using a CD-SEM extraction process, wherein the CD-SEM extraction process determines the measured CD value for a feature by measuring multiple CD values of the feature along multiple electron beam scans; determining a simulated CD value for the feature, wherein the simulated CD value is determined based at least on a set of CD extraction cut-lines which are substantially parallel to each other and are used to compute a single CD value for the feature; and during photolithography process model calibration, fitting, by computer, a parameter that models an aspect of a photolithography process based at least on both the measured CD value and the simulated CD value.