Nepali passengers with a valid passport are allowed to travel via Seoul without a transit visa, unless they need to go outside the airport. Passengers can stay in the international transit area for upto 24 hours. Until recently, they used to provide transit visa or arrival visa to Nepalis who had a valid visa for USA. They have stopped visa on arrival now.

Here are the rules on the Amadeus GDS system:

You can also call (202) 552-1508 or visit zentravels.com.

NORMAL PASSPORTS ONLY ......
TWOV (TRANSIT WITHOUT VISA):
VISA REQUIRED, EXCEPT FOR PASSENGERS TRANSITING THROUGH SEOUL
(ICN) WITH A CONFIRMED ONWARD TICKET FOR A FLIGHT TO A THIRD
COUNTRY WITHIN 24 HOURS. THEY MUST STAY IN THE INTERNATIONAL
TRANSIT AREA OF THE AIRPORT AND HAVE DOCUMENTS REQUIRED FOR
THE NEXT DESTINATION.