Development of North Sea viscous-oil fields represents a new challenge to the industry. A number of these reservoirs comprise relatively clean homogeneous sands with high horizontal and vertical permeabilities. The oil viscosities are in the range of 5 to 200 cp and oil columns are typically < 200 ft thick and often partially underlain by water. Waterflooding is dominated by gravity in the interwell region, with water slumping to the bottom of the reservoir and coning up to producers. Development depends on the use of horizontal wells to reduce the coning. The viscous oil and poor vertical sweep make these reservoirs possible targets for IOR with polymers. This study shows that polymer incremental recoveries can be high, well within the range of potential economic viability. The incremental recovery mechanism is not the usual improved Buckley-Leverett sweep efficiency resulting from correction of an adverse displacement ratio. A novel recovery mechanism is identified in which the polymer increases the ratio of viscous to gravity forces, reducing the water slumping. This leads to improved vertical sweep, recovering oil from regions of high oil saturation at the top of the reservoir. We simulated polymer flooding including temperature effects. Comparison of waterflood simulations including temperature effects with conventional isothermal simulations shows that, counter to normal expectations, isothermal simulations can underpredict recovery by > 10%.