A new Monte Carlo (MC) method is used to interpret experimental stress-optical coefficients (C) of polyethylene (PE) networks in uniaxial deformation. The method considers the deformation of individual PE-network. chains, whose conformational properties and -CH2- segment orientations are calculated using a rotational-isomeric-state (RIS) model. Individual chains, randomly oriented in three dimensions, are deformed uniaxially and affinely, but only up to the maximum chain-extension, consistent with W(r) = 0, where W(r) is the radial end-to-end distance distribution. The changes in network elastic free-energy, and in the average orientation of -CH2- segment vectors with respect to the deformation axis are evaluated. The MC method gives, in agreement with experiment, values of C that are dependent upon both deformation ratio, lambda, and on network-chain length. Furthermore, the method shows that a single equivalent freely jointed chain cannot be used to describe both stress-optical and stress-strain behaviors.