The midfielder scored twice - the first coming from the penalty spot - before leaving the field to a standing ovation after 75 minutes to be replaced by Radoslaw Majewski, who helped himself to a goal within 10 minutes of coming on.
Substitute Cedric Van der Gun netted for Swansea in the final minute but it was mere consolation for the Welsh club, who offered very little and remain pointless away from home in the npower Championship this season.
Brendan Rodgers was without midweek hat-trick hero Scott Sinclair, who was nursing a tight hamstring following the Carling Cup win at Peterborough, and as a result Swansea lacked a cutting edge in the final third.
The recalled Dexter Blackstock, the only change made by Forest boss Billy Davies, made an instant impact as he was brought down by Swansea goalkeeper Dorus De Vries after 12 minutes.
Blackstock has been on penalty duty this season but the big striker missed from the spot in the last home game against Millwall so handed the responsibility to McGugan, who made no mistake.
Swansea threatened for the first time just before the half-hour mark but Joe Allen's shot from the edge of the area was easily dealt with by Lee Camp.
McGugan blazed over the crossbar shortly afterwards before Paul Anderson, who spent the 2007/08 campaign on loan at Swansea, forced De Vries to tip his shot over after 35 minutes.
Good link-up play between Chris Gunter and Chris Cohen created the chance for McGugan to score his and Forest's second goal as he shot into the roof of the net from 12 yards out.
Substitute Andrea Orlandi sent an effort over the crossbar as Swansea made a rare foray into Forest territory midway through the second half.
Davies' side sealed victory with six minutes remaining when Dele Adebola teed up fellow substitute Majewski, who tip-toed past a couple of defenders inside the area before beating De Vries with a low shot.
That was not the end of the scoring, though, as Allen escaped down the right-hand side and crossed for Van der Gun to score from close range.