The Scarlets made the most of their numerical advantage as they ran in four second half tries to clinch a 37-3 victory that takes them two points clear of Ebbw Vale at the top of the table.
The Steelmen will get their chance to regain the top spot on Boxing Day when they travel to bottom of the table Llandovery. Other fixtures on Tuesday see Aberavon host Neath, Bedwas entertain Cardiff, Bridgend clash with Maesteg, Cross Keys face Newport and Pontypridd welcome Glamorgan Wanderers.
Llanelli fullback Morgan Stoddart ran in two tries and last season's Wales U19 outside-half Rhys Priestland helped himself to 12 points.
Priestland was also among the try scorers along with half back partner Liam Davies, flanker Jon Edwards and wing Matthew Jacobs.
Swansea's points came from the boot of replacement outside-half Stuart Thomas.