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The model for high heat-flux boiling, introduced in this paper, is based on a similarity between dropwise condensation and high heat-flux boiling, observed in simultaneous visualization experiment of liquid-solid contact and bubble structure. The bubble coalescing process in boiling on a horizontal flat surface is formulated into integral equations, assuming random positioning of dry patches and bubbles. Taking the nucleation site density and the activeness of nucleation sites, measured by image processing technique, into account, the equations are numerically solved to yield a dry patch diameter distribution as a result of cyclic process of bubble nucleation, transient bubble growth and bubble departure. The three-phase contact-line density, calculated from the dry patch distribution, has a maximum point, similar to that of experimental data. This model has a potential to explain the CHF as the extreme of smooth boiling curve, regarding wall superheat as the driving force of the CHF phenomenon.