Kathleen G. Anderson will leave after six years as president of the Greater Holyoke Chamber of Commerce July 31 for a job at Holyoke Medical Center, she said Monday. “The chamber has been probably the most rewarding job of my life. It has been so good to watch businesses grow,” said Anderson, 60. At the hospital, at 575 Beech St., Anderson will be manager of community benefits, she said. “It’s going out in the community, understanding the needs of the different parts of your community. Like going into different senior centers and asking what their needs are. I’ll also be working in the Holyoke public schools, having students talk about their interests and needs” Anderson said.