RATIONALE: Monoclonal antibodies, such as alemtuzumab, can find cancer cells and either kill them or deliver cancer-killing substances to them without harming normal cells. Giving chemotherapy drugs, such as busulfan and cyclophosphamide, before a donor stem cell transplant helps stop the growth of cancer cells. When the healthy stem cells from a donor are infused into the patient they may help the patient's bone marrow make stem cells, red blood cells, white blood cells, and platelets. Sometimes the transplanted cells from a donor can make an immune response against the body's normal cells. Giving tacrolimus and methotrexate after the transplant may stop this from happening.