Here is the full list of non-roster invites to Mets big-league camp. Pitchers and catchers report on Feb. 15. Position players report five days later.
Nimmo was the Mets' first-round pick in 2011. Plawecki was selected 35th overall in 2012 out of Purdue.
Hefner is due to miss most of the year while rehabbing from Tommy John surgery. He re-signed after being non-tendered.
Mets officials have mentioned Mazzoni, a former second-round pick out of NC State, as a potential bullpen candidate despite starting in the minors.