We have carried out Monte Carlo simulation of the motion of Ar+ ions in the space charge sheath surrounding a cylindrical Langmuir probe. The ion currents to the probe have been calculated from these simulations and the percentages of ions crossing the sheath boundary that are collected by the probe have been determined. It has been shown that the collisions of ions with neutral helium gas atoms in the sheath increase the percentage of ions collected by the probe above that predicted by collisionless orbital motion limited current (OMLC) theory at lower helium pressure and decrease this percentage below the OMLC theory prediction at higher helium pressure. It has been shown also that the ion current almost does not depend on probe radius at higher helium pressures. The results of the simulations have been compared with recent Langmuir probe measurements made in flowing afterglow plasmas and with other probe theories.