The midfielder has been impressed with his Belgian team-mate since his move from Sint-Truiden in 2011 and believes he is one of the best in his position in English football
Mignolet has made 89 appearances for the Black Cats since moving from Belgian side Sint-Truiden in 2010, and the former Aston Villa man has been impressed with his impact so far.
"I’ve been so fortunate throughout my career to play alongside some top keepers," he told the club’s official website.
I’ve played with Brad [Friedel] and Joe [Hart] and they’re great – Simon is in the same category.
"People say that Joe is the best keeper in the world; I think that Simon’s definitely up there with the best and I’m sure other people will agree.
"His saves against Arsenal were immense."
Gardner also believes the Black Cats pose more of a threat going forward after the arrival of Danny Graham from Swansea for £5 million during the January transfer window.
"Danny’s a class act," he added. "You can see when he comes onto the pitch he works his socks off and he’s there sniffing out chances and looking for goals.
"We’ve got [Steven] Fletcher up front and [Stephane] Sessegnon firing balls forward. There’s competition for places all over the pitch which is a good thing for Sunderland Football Club."