Both Conforto and Nimmo are former first-round draft picks of the Mets.
Conforto, who burst onto the scene with nine home runs in 56 games with the Mets last season, is mired in a prolonged slump, batting .107 (8-for-75) since May 24. He was the 10th overall pick in the 2014 draft.
"I think in talking with the coaching staff and the manager, we just felt that, look, this is counterproductive and what we need to do is get him to Las Vegas, get his swing back, and then hopefully get him back here within a relatively short period of time," general manager Sandy Alderson said.
Nimmo, 23, will make his major league debut Sunday. He is batting seventh and playing right field Sunday against the Atlanta Braves. He did not play in Saturday's game, which the Mets won 1-0 in 11 innings.
Mets manager Terry Collins said he will frequently use Nimmo in left field and leave Yoenis Cespedes in center field.
"It took a long time for it to set in," Nimmo said Saturday. "I was in shock. I just didn't really know exactly how to feel, because it's been something that I've been working for a long time, and I always prayed about and I always dreamt about. It's always seemed like something that I've always been reaching for. ... I've been healthy this year. That's been a big, big part."
Nimmo was the first selection of Alderson's regime (13th overall) in 2011. Nimmo was batting .328 with five home runs and 37 RBIs in 63 games at Las Vegas this season.
The 6-foot-3, 206-pound Nimmo is the earliest draft pick ever out of the state of Wyoming. Previously, it had been Michael Beaver, a sixth-round selection by the Philadelphia Phillies in 1966. There is no high school baseball in Wyoming. Montana and South Dakota are the only other states without the sport.
Collins said he came to the conclusion during Friday's game that Conforto ought to be demoted.
"I think it was after his second at-bat. He came off the field and I was just looking at him, and I could just see that he had reached the state of mental confusion," Collins said. "Just looking in his eyes, you could almost see him shaking his head, saying, 'What the heck is going on here?'
"... I don't want him to have to be scrambling his mind to figure out what he's got to change, because I don't want him to change anything. I just want him to go get some confidence and get back here."
Conforto conceded getting out of the spotlight to clear his head may be beneficial.
"It will be good to get away from the craziness of everything going on right now, just to try to simplify things and just work on getting back to who I am -- who I know I am at the plate," Conforto said. "... It's been tough. It's been very tough. It's upsetting not having the success that I know I can have."