Investors are dealing with pain this year, but Jim Simons and Renaissance Technologies LLC are racking up such huge gains that 2020 could be one of the hedge-fund firm’s best years ever.

The Long Island-based firm‘s flagship Medallion hedge fund has risen 24% this year through April 14, according to investors. The performance is thanks in part to a 9.9% gain in March, a brutal month for global stock markets. Medallion’s gains come even after its hefty investor fees, which include a cut of 36% or more of all trading gains and...