In a market with is a single seller, the competitive pressure that drives price down to the efficient level is absent. The monopolist will be able to set a price that maximizes total producer surplus.
In the graph on the left, you can see the the monopoly solution is found by drawing the marginal revenue (MR) curve and intersecting it with marginal cost.
In the graph on the right, you can play around with different price levels (the orange slider) to convince yourself that you can't improve on the monopoly optimum.