We have investigated micellization properties of surfactants using a recently developed implicit-solvent model and grand canonical Monte Carlo simulations. The original model had been parametrized for ionic surfactants at a single temperature; it is extended here to aqueous solutions of nonionic surfactants and given an explicit temperature dependence. Specifically, we have developed an implicit-solvent model of polyethylene glycol (PEG) surfactants and obtained the critical micelle concentrations (cmc's) and micellar aggregation numbers at low surfactant loadings. Various combinations of ethoxy and hydrocarbon tail segments were investigated in order to explore the predictive capabilities of the model. For the temperature dependence of,the micellization properties, we have utilized thermodynamic approaches to quantify the hydrophobic attraction at temperatures ranging from 280 to 365 K. The temperature dependence of the cmc and the aggregate sizes were obtained for various ionic and nonionic surfactants, specifically sodium dodecyl sulfate, dodecyltrimethylammonium bromide and chloride, and PEG surfactants. For all systems studied, the model yields cmc and aggregation sizes that are in near-quantitative agreement with experimental results.